Turbidimeters are well-known instruments which are used to determine the extent of turbidity in liquids (particularly water) and normally report the turbidity in terms of NTU (Nephelometric Turbidity Units). Turbidimeters determine turbidity of a liquid by measuring the extent of light scattering of a light beam projected through the liquid in a cell. Light is scattered by particles present in the liquid. Greater numbers of particles in the liquid result in greater turbidity values.
Periodically it is necessary to verify that a turbidimeter is properly calibrated. One manner of doing this is to prepare a series of standard liquid compositions of known turbidity and then taking turbidity measurements of each standard composition. By comparing the instrument reading with the known turbidity value of each standard composition, it is possible to verify whether the instrument is properly calibrated. However, this is a very time-consuming, tedious and expensive procedure.
A commercially available device which has been used for verifying the calibration of a turbidimeter is a solid glass cube which has been doped with a material having a refractive index slightly different from that of the glass itself. When a light beam in the turbidimeter passes through the glass cube it is scattered by the dopant material. The scattered light forms a signal at the detector in the instrument. There are a number of disadvantages associated with the use of this technique, including: (a) the turbidimeter must be dried and cleaned before the cube can be inserted, (b) the cost of the cube is very significant, and (c) it is not possible to manufacture cubes which provide exactly the same light scattering value (consequently each cube must be individually certified by the manufacturer).
There has not heretofore been provided a calibration verification system having the features and advantages provided by the present invention.